I don't have the dyno scanned YET but first the bad news.

You DO loose low end torque from a free flowing exhaust!

From 2,500 to 3,500 rpms I lost almost exactly 4 ft/lbs of torque

From 3,500 to 5,500 rpms I lost almost exactly 2 ft/lbs of torque.

I would speculate that with a cat/back 2.5" piping you would loose even more low end/mid range torque.

Now for the good news...

From 5,500 to 6,500 my peak power increased by 7.5 hp!!! (This is all SAE corrected hp and torque.

My torque curve also is about 6-8 ft/lbs higher after 5,500 all the way to 6,500rpms.

Please remember, I already had intake and headers before adding the exhaust, so I'm pretty positive someone with no mods would get much lower gains.

Here is the wierd part.

My baseline dyno run with the stock exhaust was 154.2hp and 145.9 ft/lbs torque (My last dyno from March after I got the denso's was 171 hp and 160 ft/lbs but this was with my stock wheels)

After I put on the CompTech exhaust my car dyno'd 161.7 hp and 144.1 ft/lbs torque. It seems like the more miles I put on my car the more HP I lose.

 

I think this is normal with day to day driving, especially with the wear and tear factor. The gunk that collects over miles and many other things. Bimmer magazine was talking about this with a long term 328i.